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Accepted Manuscript
Synthesis of phosphotungstic acid-supported bimodal mesoporous silica-based
catalyst for defluorination of aqueous perfluorooctanoic acid under vacuum UV
irradiation
Xia You, Lin-ling Yu, Fang-fang Xiao, Shi-chuan Wu, Cao Yang, Jian-hua
Cheng
PII:
DOI:
Reference:
S1385-8947(17)31838-7
https://doi.org/10.1016/j.cej.2017.10.123
CEJ 17907
To appear in:
Chemical Engineering Journal
Received Date:
Revised Date:
Accepted Date:
3 July 2017
22 September 2017
19 October 2017
Please cite this article as: X. You, L-l. Yu, F-f. Xiao, S-c. Wu, C. Yang, J-h. Cheng, Synthesis of phosphotungstic
acid-supported bimodal mesoporous silica-based catalyst for defluorination of aqueous perfluorooctanoic acid under
vacuum UV irradiation, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/j.cej.2017.10.123
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Synthesis
of
phosphotungstic
acid-supported
bimodal
mesoporous silica-based catalyst for defluorination of aqueous
perfluorooctanoic acid under vacuum UV irradiation
Xia You a, Lin-ling Yu a, Fang-fang, Xiao a, Shi-chuan Wu a, Cao Yang a, Jian-hua
Cheng a, b*
a
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,
Ministry of Education, School of Environment and Energy, South China University of
Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR
China
b
South China Institute of Collaboration innovation, Dongguan, 523808, China
*Corresponding author. Address: College of Environment and Energy, South China
University of Technology, Guangzhou 510006, China. Tel.: +86 13926448812; fax:
+86 20 38743651
E-mail address: jhcheng@scut.edu.cn (J.-h. Cheng)
Other authors
Xia You (E-mail address: 840957540@qq.com)
Lin-ling Yu (E-mail address: 1943406346@qq.com)
Fang-fang Xiao (E-mail address: 1447664536@qq.com)
Shi-chuan Wu (E-mail address: 492754075@qq.com)
Cao Yang (E-mail address: 997786113@qq.com)
1
Abstract: Phosphotungstic acid (H3PW12O40, HPW)/bimodal mesoporous
silica
(BMS) multifunctional catalysts were prepared and employed as photocatalyst for
perfluorooctanoic acid (PFOA) degradation under vacuum ultraviolet light (VUV).
Lower PFOA concentration, appropriate HPW/BMS dosage (0.2 g·L-1) and acidic
condition (pH = 3 ~ 4) were more favorable for PFOA defluorination in
VUV/HPW/BMS system. Furthermore, we analyzed the intermediates produced
during PFOA decomposition by HPLC/MS/MS, which were shorter-chain
perfluorinated carboxylic acids (PFCAs) with C7, C6, C5, C4, C3 and C2. The
defluorination mechanism of PFOA was proposed, i.e., direct photolysis played an
important role in PFOA defluorination under VUV radiation (λ= 185 nm), while the
defluorination of PFOA was simultaneously photocatalytically decomposed and
defluorinated in the presence of HPW/BMS (λ = 254 nm).
Keywords
Bimodal mesoporous silica; Phosphotungstic acid; PFOA; VUV irradiation;
Defluorination
2
1. Introduction
Perfluorooctanoic acid (PFOA), an emerging class of environmental contaminants
presented in various environmental and biological matrices, has received much
attention [1, 2]. PFOA has been revealed to have potential toxicity on human organs,
reproductive and immune systems, mainly in the performance of liver toxicity [3, 4],
embryonic developmental toxicity [5], endocrine disruption [6], neural toxicity [7], and
potential carcinogenicity [8, 9]. Recently, PFOA has been frequently detected in surface
water [10-12], grounder water [13], wildlife species [14, 15], even humans [16, 17]. It
is crucial to explore effective methods for eliminating PFOA from environment.
PFOA contains high energy carbon–fluorine covalent bonds [18], which makes it
resistant to environment degradation [19]. It is quite difficult to achieve a total
restriction of PFOA under chemical and thermal destructions in global [20]. According
to previous reports, traditional wastewater treatment methods, such as biodegradation
and chemical degradation are unable to eliminate PFOA effectively [21, 22]. In recent
years, ultrasonication method [23, 24], electrochemical methods [25] and photo
-Fenton [18] have been proved to show high efficiency on the degradation of PFOA.
However, they are restricted by the high energy consumption and strict reaction
conditions. Therefore, it is increasingly urgent to develop promising treatment
techniques to transform PFOA into harmless species under mild conditions.
Photocatalysis and photochemistry are popular for their low energy consumption
to degrade the contaminants in mild condition [26-29]. TiO2 is a widely used
photocatalyst, but it is easy to produce TiO2 surface fluorination due to the
photodegradation of PFOA, leading to TiO2 inactivation [30]. Hori et al. [31] utilized
tungstic heteropolyacid (H3PW12O40, HPW) photocatalyst to achieve efficient PFOA
decomposition and the production of F- ions and CO2, the PFOA disappeared
completely within 24 h under UV irradiation (254-nm, 200-W xenon-mercury lamp) in
the presence of HPW, without accompanying catalyst degradation. HPW is an attractive
candidate catalyst in PFOA degradation due to its multielectron redox capabilities, high
3
stability, and super-duper UV absorption properties. However, it is worth noting that
HPW has small specific surface area [32], limiting its application to a large extent. A
number of attempts have been made to develop efficient catalysts with HPW dispersed
on supports such as SiO2 [33], MCM-41 [34], SBA-15 [35] and bimodal mesoporous
silica (BMS) [36]. BMS forms a bimodal mesoporous structure during the synthesis
process, which is beneficial to the diffusion and reaction of the reactant molecules in
the pores, and has great potential in the macromolecule catalytic reaction. Considering
that the specific surface area of BMS (1000 m2·g−1) is larger than that of SiO2 (400
m2·g−1) and the like, it is a promising support capable of providing sufficient inner and
outer surfaces for the dispersion of HPW. It is expected that the catalyst with abundant
strong acid sites can be obtained by loading HPW on BMS. In addition, its strong
thermal stability is also very attractive [37]. Then, PFOA degraded very slowly under
irradiation of 254 nm UV light, but effective degradation under 185nm vacuum
ultraviolet (VUV) irradiation [38]. The decomposition of PFOA will be highly
conducted with VUV irradiation in the presence of bimodal mesoporous
phosphotungstic acid silica catalyst (HPW/BMS).
In this work, a series of highly uniformed BMS functionalized with HPW were
synthesized and characterized. The structures, morphologies, and optical properties
were investigated in detail. Then the defluorination activities of VUV/HPW/BMS
system on the degradation of PFOA were evaluated sufficiently, including the effects
of initial PFOA concentration, HPW loadings, HPW/BMS dosage and solution pH.
Subsequently, the intermediates produced during PFOA photolysis were qualitatively
and quantitatively analyzed by HPLC/MS/MS, and the degradation mechanism of
PFOA was also clarified.
2. Materials and methods
2.1 Chemicals
PFOA (C7F15COOH, 96%), perfluoroheptanoic acid (C6F13COOH, 98%) and
4
12-tungstophosphoric acid (H3PW12O40, 99.9%) were purchased from Aldrich
(Germany). Perfluorohexanoic acid (C5F11COOH, 98%) and perfluoropentanoic acid
(C4F9COOH, 98%) were gained from Tokyo Kasei (Japan). Perfluorobutanoic acid
(C3F7COOH, 99%) and trifluoroacetic acid (CF3COOH, 97%) were obtained from
Adamas Reagent (Switzerland). Perfluoropropionic acid (C2F5COOH, 97%) was
bought from Acros Organics, (Belgium). Tetraethyl orthosilicate (TEOS, 28%),
cetyltrimethyl ammonium bromide (CTAB, 99%) and ammonia (NH4OH, 25%) were
obtained from Guangzhou Chemical Reagent Factory (China). All of the reagents were
used as received and ultrapure water supplied by the Millipore Milli-Q ultrapure water
system (USA) was used throughout the subsequent reaction.
2.2 Preparation of phosphotungstic acid - supported catalyst (HPW/BMS)
HPW/BMS was synthesized using two-step synthesis as outlined in reference [37].
Firstly, TEOS, CTAB, water, and ammonia were mixed in a ratio of 1:0.2:160:1.5 at
room temperature, the formation of homogeneous sol could be immediately observed.
With continuously stirring, the solution became more viscous and eventually turned
into a white gel. This gel was filtered and repeatedly washed with deionized water, then
dried and subsequently calcined in air by heating at a ramping rate of 5 °C /min to
550 °C and maintained this temperature for 6 h, leading to the BMS.
Secondly, HPW solution was prepared beforehand with the desired amount of
HPW (0.1, 0.2, 0.3 and 0.4 g) dissolved in 10 mL deionized water. Then, 1.0 g of BMS
was successively added into the above solution under vigorous stirring. Nitric acid was
dropped into the mixed solution until pH less than 1. The resulting mixture was stirred
for 12 h at room temperature, and subsequently dried under vacuum at 110 ℃ overnight,
a gray powder was obtained. The powder was sent for calcination in a furnace at 200 ℃
for 2 h, after which the resulting sample was kept in a desiccator. The catalysts are
denoted as M wt%-HPW/BMS in which the value of M could be 10, 20, 30 and 40.
2.3 Characterizations
The
crystal structure of catalysts was characterized by XRD
5
with
monochromatized Cu Ka radiation (Bruker D8 Advance). The detection of specific
chemical bonds within the catalysts was achieved by FTIR (Nicolet 5700 spectrometer)
with KBr pellets. SEM (Nova Nano 450, FEI) was used to explore the morphology.
Nitrogen adsorption-desorption isotherms were conducted on an instrument
(Micromeritics ASAP2020). Pore size distribution was analyzed by non-local density
functional theory (NLDFT). The chemical composition of the samples was investigated
by XPS (Thermo ESCALAB 250XI) with an Al Ka source. The UV–vis spectrometer
(Varian Cary 500) was employed to record the diffuse reflectance spectra of the
catalysts.
2.4 Degradation experiments
Photocatalytic defluorination reactions were carried out in a Teflon cylindrical
reactor with an inner diameter of 75 mm, in which a magnet rotor was placed. A
low-pressure mercury lamp (8 W, Philips Lighting Co., Guangdong, China), emitting
mainly 254 nm with a small amount of 185 nm VUV light, was used as the irradiation
source and placed in the center of the reactor with a quartz tube protection. The
reaction solutions of PFOA were freshly prepared before the experiment and used
immediately. In a typical experiment, a 500 mL mixture of PFOA (5~40 mg L-1) and
HPW/BMS were added into the reactor. In addition to the single factor test of pH, the
pH was not adjusted. No additional gas was provided throughout the reaction period.
The whole photochemical reactor was placed in a standing-temperature cultivator, and
kept at room temperature in all reactions. At regular time intervals, samples were taken
out for subsequent analysis.
2.5 Analysis
PFOA and the intermediates were quantitatively analyzed by LC/MS/MS. Fconcentration in aqueous solution was quantified by an ion-chromatography system
(ICS-90, Dionex). The detection conditions were detailed in the supplementary data.
Defluorination ratio of PFOA was calculated as follows:
6
Deluorinationratio =
×
× 100
(1)
Where CF- is the concentration of fluoride ion, mM, C0 is the initial concentration
of PFOA, mM. The factor of 15 corresponds to the number of fluorine atoms in a
PFOA molecule. The regression coefficient (R2) of the calibration curves was 0.9999.
3. Results and discussion
3.1 Characterizations
Fig.1a displays the XRD patterns of BMS and HPW/BMS multifunctional catalyst.
For catalysts with 10wt% and 20wt% loadings, no obvious separated HPW phase was
observed, indicating that HPW was finely dispersed on BMS. As the loading amount of
HPW gradually increased (30wt%) and reached 40wt%, the characteristic peaks
became more obvious and sharper. It is also found that the diffraction pattern of
HPW/BMS were nearly the same as that of the HPW previously reported in the
literature [34]. In addition, these catalysts all exhibited diffraction peaks at 2θ = 2°~3°,
corresponding to (100), and the position of the (100) peak shifted to a lower angle
compared to BMS may due to the pore expansion. Meanwhile, no signals of the
higher order peaks (110) and (200) were detected, demonstrating the absence of a
hexagonal pore ordering [37].
The FTIR spectra (600~1200 cm-1) for pure HPW, BMS, and HPW/BMS were
shown in Fig. 1b. The characteristic bands of HPW were detected, including the
vibrations of asymmetric P=O at central tetrahedral at 1087 cm-1, terminal asymmetric
oxygen (W=Od) at 989 cm-1, corner shared asymmetric oxygen (W-Ob-W) at 883 cm-1,
and edge shared oxygen (W-Oc-W) at 802 cm-1 [39]. With the increase of HPW loading
amount, the bands at 989, 883and 802 cm-1 indicative of Keggin unit were strengthened.
This suggests that the Keggin structure of HPW introduced into the support was
retained [40]. In comparison with the standard characteristic peaks of HPW, obvious
red shift of W=Od band could be identified. For pure HPW, a band of W=Od at 989
cm-1 was detected, whereas that shifted to 981 cm-1 for 30wt%-HPW/BMS and
7
40wt%-HPW/BMS, 979 cm-1 for 20wt%-HPW/BMS, and hardly identified for
10wt%-HPW/BMS due to the masking effect of silica. These red shifts could be used
as evidence of the interaction between HPW and BMS. In specific, the Keggin
structure of HPW anion was electron-rich polyoxoanions, and thus acts as strong
electron donor to supply electrons to Lewis sites through the terminal W=Od bonds
[39].
Fig. 1. (a) XRD patterens of BMS, HPW and HPW/BMS. The inset is the low-angle XRD
patterns of HPW/BMS. (b) FTIR spectra of BMS, HPW and synthesized catalysts.
Fig. 2 presents the SEM images of BMS and all HPW/BMS. We can see from Fig.
2a that BMS showed a granular structure with the particle size of less than 50 nm. For
HPW/BMS, similarity in shape and surface morphology with BMS were observed (Fig.
2b-e). This indicates that the introduction of HPW almost had no effect on the
morphology of BMS. There was no visible presence of HPW on the surface,
indicating that HPW was well dispersed on the surface of BMS. With increasing the
HPW loading amount, the particle size of HPW/BMS decreased slightly, the tendency
of agglomeration reduced significantly, and the dispersion became better. This may be
related to the presence of polar groups, such as Si-OH on the BMS, which is prone to
self-agglomeration. The additional HPW may interact with Si-OH, thus preventing the
occurrence of agglomeration.
8
(a)
(c)
(b)
50nm
50nm
50nm
(e)
(d)
50nm
50nm
Fig. 2. SEM images of (a) BMS, (b) 10wt%-HPW/BMS, (c) 20wt%-HPW/BMS, (d)
30wt%-HPW/BMS, (e) 40wt%-HPW/BMS.
Fig.3 shows the N2 adsorption-desorption isotherms of BMS and HPW/BMS. As
previously reported [40], the synthetic materials could be considered as mesoporous.
The isotherms of all the materials exhibited the standard Type IV isotherm according to
the IUPAC classification, characterized by a step increase from relative pressure of 0.2
to 0.35 due to capillary condensation. With a closure at around P/Po = 0.4, all of them
had narrow hysteresis loop (hysteresis type H1), when the relative pressure within
0.7~0.9, the second hysteresis loop occurred. In general, all HPW/BMS had similar
isotherms in terms of hysteresis shape as compared with BMS. However, for higher
loading HPW/BMS, the drop in intensity could reflect the fact that increased amounts
of smaller pores compared to that at lower loading [39]. From the pore size distribution
analysis (Fig. 3 inset), BMS and 30wt%-HPW/BMS all exhibited mesoporous centered
at about 3 and 16 nm, respectively. All samples synthesized had high surface area in the
range of 566~896 m2·g-1, and the parameters calculated from the isotherms were listed
in Table 1. It is noted that BET surface areas gradually decreased with the increasing of
HPW loading amounts, which may be attributed to the partial blocking by the large
amounts of kegging units.
9
Table 1 Surface characteristics of BMS and the HPW/BMS samples
Sample
Surface area
Pore volume
(m2 g-1)
(cm3 g-1)
BMS
896
0.92
10wt%-HPW/BMS
840
0.87
20wt%-HPW/BMS
726
0.75
30wt%-HPW/BMS
662
0.68
40wt%-HPW/BMS
566
0.57
Fig. 3. Isotherm profiles of BMS, 10wt%-HPW/BMS, 20wt%-HPW/BMS, 30wt%-HPW/BMS,
and 40wt%-HPW/BMS. The inset shows the pore size distribution of BMS and
30wt%-HPW/BMS.
To analyze the interaction between Keggin unit and BMS, high-resolution XPS
analysis was conducted for the representative samples. Full spectrum of both typical
materials (BMS and 30wt%-HPW/BMS) were observed in Fig. 4a. It can be seen that
BMS had the O, C and Si elements. After the addition of the HPW, the P and W
elements were detected in the XPS spectra, the composition of each element was
described in the supplementary data. Fig. 4b exhibits the high resolution W4f XPS
spectra of 30wt%-HPW/BMS, where two peaks at 36.3 and 38.5 eV can be assigned
to W4f7/2 and W4f5/2, respectively. Compared to the HPW, a slight shift in positive
direction was observed in 30wt%-HPW/BMS, indicating a change in chemical
10
binding of W. The case of shifting to higher binding energy also reflected a decrease
in the electron density of W, suggesting that electron transfer from HPW to BMS
surface [41]. This phenomenon was in good accordance with the FT-IR spectrum. Fig.
4c demonstrates the O1s XPS lines of BMS, pure HPW, and 30wt%-HPW/BMS.
BMS shows one peak centered at 533.7 eV, originating from the Si-OH. The O1s XPS
line of pure HPW displays one peak at 530.7 eV. After incorporation of the Keggin
unit into BMS, two peaks at 530.8 and 533.3 eV were detected, Where O 1s peak of
Si-OH migrated in the negative direction. The shift of O 1s peak was attributed to -OH
groups substitution and covalent Si-O-W bonds formation. According to above
analysis, the electron transfer between the components on the surface of HPW/BMS
can be deduced. HPW acts as electron donor assisting in the generation of chemical
bond between HPW and surface silanols on BMS.
Fig. 4. (a) XPS spectra of BMS, HPW and 30wt%-HPW/BMS, (b) High resolution W 4f XPS
spectra of HPW and 30wt%-HPW/BMS, and (c) High resolution O 1s XPS spectra of BMS, HPW
and 30wt%-HPW/BMS.
Besides the structural and texture properties, optical property of HPW/BMS was
also significantly affected by the HPW modification. The bare HPW exhibited good
light absorption performance in the range of 200~415 nm. Light absorption of HPW
was attributed to charge transfer response from O2p to W5d orbit at W=O and W-O-W
bonds, respectively [42]. As shown in Fig. 5, BMS presented little visible light
absorption in the range of 200~400nm. However, after the incorporation of HPW, the
light-harvesting capability of 30wt%-HPW/BMS remarkably enhanced in the region
11
ranging from 200 nm to 400 nm, which is due to the intense light absorption ability of
the HPW clusters. And the improved UV light absorption performance would be
beneficial to the enhancement of photocatalytic activity of the HPW/BMS.
Fig. 5. UV-vis spectra of BMS and 30wt%-HPW/BMS.
3.2 Photocatalytic test
3.2.1 Effects of VUV/BMS/HPW photolysis system on photodegradation of PFOA
It has been established that PFOA could be directly decomposed by VUV
radiation (<200 nm), as PFOA had strong absorption in 185 nm VUV light region [43].
The PFOA solution with initial concentration of 10 mg·L-1 was degraded by VUV,
VUV/BMS, VUV/HPW and VUV/HPW/BMS system, respectively, in which the
dosage of BMS, HPW and HPW/BMS were 0.2 g·L-1. As shown in Fig. 6a, with the
degradation of PFOA, the amount of fluoride ions in the solution was gradually
increased, the defluorination rate under irradiation with 185nm VUV light was reached
19%, slightly higher than that of the VUV/BMS system. Thus the effect of BMS in the
system was negligible and almost no absorption was observed for F-. Compared with
direct photolysis under VUV (185nm) irradiation, the degradation effect of PFOA was
improved in both VUV/HPW system and VUV/HPW/BMS system. In the presence of
pure HPW, approximately 25% of PFOA was defluorinated after a 4-h treatment.
However, with the addition of HPW/BMS, the defluorination process was greatly
accelerated. The defluorination rate (50%) was more than three times the rate obtained
by direct photolysis. Therefore, it could be concluded that the addition of HPW/BMS
12
favors the rapid photochemical defluorination of PFOA. An average of 7.53 fluorine
atoms contained in PFOA were converted to fluoride ions, which further demonstrated
that the coexistence of HPW/BMS and VUV irradiation would achieve more efficient
photodegradation of PFOA.
The PFOA decomposition well fitted the first-order kinetics, as shown in Fig. 6b.
The corresponding reaction rate constants for the defluorination of PFOA in the VUV,
VUV/BMS, VUV/HPW and VUV/HPW/BMS systems were 5.6, 5.0, 8.0, 18.1 × 10-4
min-1, respectively. The defluorination rate constant in the presence of HPW/BMS was
approximately 3.2 times higher than that observed by direct photolysis, and about 2.3
times in the presence of phosphotungstic acid. It illustrated that there was
photocatalytic degradation of PFOA in addition to direct photolysis of PFOA,
HPW/BMS was mainly to improve the efficiency of UVC (254 nm) utilization, then
enhance the degradation rate of PFOA and intermediates.
Fig. 6. (a) Photochemical defluorination efficiency and (b) fitted pseudo-first-order kinetic curve
under VUV, VUV/BMS, VUV/HPW and VUV/HPW/BMS systems.
3.2.2 Effects of HPW loading amounts on BMS
In this study, effects of HPW loading (between 10 and 40wt%) in the catalysts on
photodegradation of PFOA were investigated, the other reaction variables such as
reaction temperature, function time, initial concentration were fixed at 25℃, 4 h, 10
mg·L-1, respectively. In Fig. 7a, HPW/BMS presented the highest conversion (52%)
13
when the mass ratio of HPW was increased further to 3:10, in contrast,
10wt%-HPW/BMS showed the lowest activity, with the defluorination rate of 24%.
And with higher HPW anion loadings (40wt%), HPW/BMS demonstrated decreased
activity compared with 30wt%-HPW/BMS and the defluorination efficiency was
declined to 42%. Meanwhile, the defluorination rate constant values also reduced to
1.52 × 10-3 min-1 (Fig. 7b).
Through the above study we could see that catalysts with incorporation of the
Keggin unit were more conductive to enhance photocatalytic quantum efficiency. As
the HPW loading rate increased from 10wt% to 30wt%, the defluorination rate
enhanced accordingly. This could be simply explained that with increasing amount of
catalyst available in the system, more acid site was available to catalyze the reaction.
Thus, better defluorination could be observed. The activity of 40wt%-HPW/BMS was
reduced as compared to 30wt%-HPW/BMS. This was due to their mesoporous were
significantly blocked at HPW loadings higher than 30wt%, resulting in the inability of
acidic
sites
embedded
in those
mesoporous during
the
reaction.
Thus,
30wt%-HPW/BMS demonstrated quite promising results as compared to others
investigated above. Its dual pore structure and relatively larger internal mesoporous
would allow significant access of reactants to the active sites while at the same time
playing a role in providing sufficient catalytic sites.
Fig. 7. (a) Effects of HPW loading on BMS on the defluorination of PFOA and (b) fitted
14
pseudo-first-order kinetic curve.
3.2.3 Effects of initial PFOA concentration
In order to clarify whether PFOA concentration significantly affects UV
photodegradation, five samples with initial PFOA concentrations of 5, 10, 20,30 and
40 mg·L-1 were examined while the rest of process variables were fixed (room
temperature, 30wt.% HPW loading, 0.2 g·L-1 catalyst and continuous stirring).
Apparent that the defluorination rate values decreased gradually with the increase of
concentration, which were 53, 50, 44, 33 and 30% respectively (Fig. 8a). The
defluorination rate constant values with the five concentrations identified above were
approximately 1.89, 1.81, 1.44, 1.11, 0.98 × 10-3 min-1, respectively (Fig. 8b). These
observations suggest that the PFOA defluorination rate appeared to be negatively
correlated with the initial concentration in the studied concentration range.
PFOA was mineralized in a muti-step manner with the formation of several short
carbon-chain fluorinated compounds (PFCAs) as intermediates in the process of UV
photodegradation. However, it is uncertain if the physicochemical characteristics of
PFACs affect their defluorination efficiencies [43]. Naturally, a lower rate of
defluorination would occur, mainly due to the effective photolysis of PFCs is critical
for strengthening defluorination [44]. In addition, the light density generated at a
specific UV power was constant, a photon-limited phenomenon may arise when the
initial PFOA concentrations was relatively high, resulting in slow PFOA elimination.
PFOA could achieve effective direct photolysis and photocatalytic degradation at low
concentration by providing abundant photons. However, in the case of relatively high
PFOA concentrations, with the supply of photons running out, the excess PFOA could
not achieve direct photolysis nor photocatalytic degradation. Meanwhile, this process
may also delay the photodecomposition of other short carbon-chain byproducts,
resulting in further lower defluorination efficiencies. Furthermore, HPW/BMS dosage
had significant influence in the photocatalytic degradation of PFOA. In this section,
the dosage of HPW/BMS was 0.2g·L-1, and the dosage might be too small to achieve
15
efficient photocatalytic defluorination of PFOA, especially at high initial PFOA
concentration. In summary, PFOA concentration (varying from 5 to 40 mg·L-1) has a
great impact on its mineralization and deserves further exploration of a more rational
explanation for lower defluorination efficiencies with larger PFOA concentrations.
Fig. 8. (a) Effects of initial PFOA concentration on the defluorination of PFOA and (b) fitted
pseudo-first-order kinetic curve.
3.2.4 Effects of dosage
Next, the effects of 30wt%-HPW/BMS dosage (varying from 0.1 to 1.0 g·L-1) on
the defluorination of PFOA were studied, the PFOA concentration were 10 mg·L-1
while maintaining the rest experimental parameters. As shown in Fig. 9(a), increasing
HPW/BMS dosage led to corresponding increases in defluorination within a certain
dosage range. Specifically, the defluorination ratio of PFOA at 4 h reached 29% in the
presence of 0.1 g·L-1 HPW/BMS, and the defluorination ratio of PFOA greatly
increased to 50% when the HPW/BMS concentration was increased to 0.2 g·L-1.
However, the defluorination ratio of PFOA decreased gradually once the dosage of
HPW/BMS was more than 0.2 g·L-1. Moreover, with the change of HPW/BMS
dosage, the defluorination rate constant values in the presence of various HPW/BMS
concentration were approximately 1.03, 2.04, 1.36, 1.31 and 1.32×10-3 min-1 (Fig. 9b).
Results indicate that superfluous HPW/BMS might adversely affect the defluorination
of PFOA.
From the photochemical principle, we could know that a photon-limitation
16
phenomenon existed at high HPW/BMS concentrations when UV power was constant.
In practical photodegradation reactions, photons could be directly absorbed by PFOA,
and other reactions may influence the consumption rate of the target reactant. When
the HPW/BMS concentration was low (<0.2 g·L-1), the activity per unit volume
increased with further addition of HPW/BMS, and the defluorination rate increased
accordingly. Then, the tendency for PFOA defluorination rates and their constant
values were to decrease at relative high HPW/BMS concentrations, this may be
attributed to scattering of the light: when a lot of catalyst particles were present in the
reaction environment, the light adsorption of the particles near the source shaded the
others behind, thereby hindering the generation of more photoproduction holes, and
causing the decrement of direct photolysis and photocatalytic degradation efficiency.
Moreover, the limited amounts of photons would also be detrimental to the progress
of the reaction. There is a saturation point for HPW/BMS dosage in the studied
system: in our experiment condition, the HPW/BMS dosage of 0.2 g·L-1 is most
beneficial for improving the defluorination of PFOA, and selected in the following
researches.
Fig. 9. (a) Effects of HPW dosage on the defluorination of PFOA and (b) fitted pseudo-first-order
kinetic curve.
3.2.5 Effects of initial pH
In VUV/HPW/BMS system, to further explore the possible pathway that PFOA
decomposes in different conditions, photodegradation of PFOA was evaluated at five
17
initial pH values (2, 3, 4, 5 and 7). The initial pH values were maintained by adjusting
the pH of PFOA solution (10 mg·L-1) using standard NaOH and HCl solutions. The
defluorination rates and the fitted pseudo-first-order kinetic curves under different
initial pH values were shown in Fig. 10. The defluorination effect was different
depending on the initial pH. The defluorination rate of PFOA gradually increased up
to 38% when the pH value increased from 2 to 4. However, the PFOA defluorination
rates and their constant values were drastically decreased when pH further increased
to a neutral environment, which were only 7% and 0.18 ×10 -3 min-1 at a pH of 7.
PFOA could be, by contrast, effectively decomposed and defluorinated at pH values
of 3 and 4.
According to previous study of other organic compounds, pH value may have an
important influence on PFOA defluorination due to the quantum yields, pKa values of
PFOA and the intermediates produced during defluorination process [45, 46]. The
pKa value for PFOA in water is 2.8 [47]. Under acidic conditions, PFOA mainly
exists in molecular state (C7F15COOH), while it appears in ionic state (C7F15COO-) in
neutral and alkaline conditions. The great enhancement of PFOA photodegradation
observed in acidic pH conditions was attributed to the predominance of PFOA’s
molecular state in the bulk solution. It has been reported that PFOA could exist in the
protonated form when the pH of the action solution is lower than 2.8, at this point
PFOA decarboxylation process would be blocked, which was not conducive to
electronic transfer. Therefore, heavily acidic and alkaline conditions are not beneficial
to the defluorination of PFOA, acidic conditions (pH=3~4) are the most appreciate.
18
Fig. 10. (a) Effects of initial pH on the defluorination of PFOA and (b) fitted pseudo-first-order
kinetic curve.
3.3 Defluorination intermediates
In order to further explore the specific process of PFOA defluorination, the
detection of intermediates was achieved by HPLC/MS/MS. Six different shorter-chain
PFCAs, including PFHpA, PFHxA, PFPeA, PFBA, PFPrA and TFA were identified
and quantified in the PFOA reaction solution. The TIC spectra of PFCAs produced
during the reaction process were presented in supplementary data. The degradation
intermediates are the same as those reported by other researchers [38, 44, 48]. In Fig.
11a, the degradation of PFOA and the formation of intermediates over the irradiation
time were presented. It was clear that the amount of PFCAs with longer carbon chains
(PFHpA, PFHxA) grew faster and the presence of shorter-chain PFCs was almost
undetectable within 1 h. After 4 hours, the concentration order of PFCs was
PFHpA>PFHxA >PFPeA > PFBA > PFPA > TFA. The concentration of PFHpA and
PFHxA in the solution were much higher than that of short-chain PFCs. These
observations clearly indicate that PFOA degrades to long-chain PFCs at the beginning
of reaction, followed by a step-by-step transformation of long-chain PFCs into other
intermediates with shorter chains.
Quantitative analysis based on total fluorine indicated that the fluorine was
present in three states, namely: organic fluorine, F- and unknown fluorine. The
organic fluorine was derived from the undegraded PFOA and the intermediates The
19
mass concentration was calculated by the following formula:
Y = ∑%&(3 + 2) × ! ( " )#$$
(2)
The mass concentration of F- in the solution was analyzed by ion
chromatography at different reaction times. The unknown fluorine could be
considered as the part of the fluorine that was not identified in the above analyses.
The proportion of fluoride in various states was shown in Fig. 11b, the proportion of
F- was almost the same as that of the lost organic fluorine, which indicated that the
organic fluorine lost during the degradation process might be converted to F-. In
addition, we found that organic fluorine and F- accounted for more than 87 % of the
fluorine in the decomposed PFOA solution at any point during the reaction process.
Since the percentage of unknown fluorine was low, we could conclude that
shorter-chain PFCs were the major intermediates of PFOA in the degradation of
VUV/HPW/BMS system.
Fig. 11. (a) Evolution of the PFOA and the intermediates decomposition in VUV/HPW/BMS
system and (b) fluorine element mass balance during PFOA decomposition.
3.4 Defluorination mechanism
On the basis of the results mentioned above, two pathways that PFOA
decomposes under VUV/HPW/BMS system in different conditions were proposed.
One of which is direct photolysis under VUV irradiation. As previously reported in
literatures [38, 44], PFOA could be excited by 185-nm VUV light, resulting in a
20
decarboxylation reaction to produce C7F15 and COOH radicals by breaking the C–C
bond between the C7F15 and COOH (4). Then, the C7F15 radical was immediately
hydrolyzed and transformed into the unstable form C7F15OH (5), the unstable
C7F15OH was finally converted to C6F13COOH (PFHpA) and F- (6,7). The
intermediate C6F13COOH further degraded into a series of PFCs in a similar pathway.
PFOA + ℎ+ → C. F · + ·C001
(3)
C. F· + H3 O → C. F OH + H
(4)
C. F OH → C% F4 COF + F 5 + 1 6
(5)
C% F4COF + H3 O → C% F4 COOH + H6 + F 5
(6)
Another approach was initiated by H3PW12O40 under 254 nm UV irradiation [31,
49]. Under UV light irradiation, the ground-state species [PW12O40]3- was excited to
excited-state [PW12O40]3-* radicals (7). An electron transferred from PFOA to the
excited-state species (8), and [PW12O40]4− was reoxidized to [PW12O40]3− in the
presence of oxygen (9). When the oxygen content was small, the reoxidation process
was very slow. The C-C bond associated with the carboxyl group cleaved to produce
C7F15 radicals and CO2 (10). The C7F15 radicals in the solution could be converted to
C7F15O2 in the presence of oxygen (11). Then, the C7F15O2 radicals in water could be
decomposed to C7F15O radicals (12). C7F15O radicals decomposed by two pathways,
one was to form C6F13 radicals and CF2O (13), another pathway was achieved through
reacting with HO2 (14), and eventually generated C6F13COOH (5, 6). The
intermediates produced in both pathways were further transformed into shorter chain
PFCs in a similar manner, respectively.
7PW3 O9& :45 + ℎ+ → 7PW3 O9& :45∗
(7)
7PW3 O9&:45∗ + PFOA → 7PW3 O9& :95 + PFOA6
(8)
7PW3 O9& :95 + O3 → 7PW3 O9& :45 + 03 5
(9)
C. F OO· → C.F · + CO3
(10)
C. F · + O3 → C. F OO·
(11)
2C. F OO· → 2C. F O· + O3
(12)
21
C. F O· → C% F4· + COF3
(13)
C. F O· + HO3 → C. F OH + O3
(14)
As shown in Figure. 12, PFOA could be degraded and defluorinated to
shorter-chain PFCAs through direct photolysis (VUV 185-nm light) and
photocatalytic degradation in a stepwise way. PFOA and the intermediates could be
completely mineralized to CO2 and fluoride ion for a sufficient period of time.
Fig. 12. Proposed mechanism for the decomposition of PFOA in VUV/HPW/BMS system.
4. Conclusions
The attempt in utilizing HPW combined with high surfaced bimodal mesoporous
silica materials were made before PFOA photodegradation reaction. Characterization
results showed that HPW was successfully introduced and dispersed well on BMS,
the 30wt%-HPW/BMS was proved to be the best catalyst. The VUV/HPW/BMS
system was shown to significantly enhance the defluorination rates of PFOA compared
to VUV alone, especially in the low PFOA concentration, appropriate HPW/BMS
dosage and acidic condition (pH = 3 ~ 4). Moreover, analysis by LC/MS/MS revealed
that there were six short-chain PFCs containing C2-C7 perfluoroalkyl groups and were
formed sequentially. Both direct photolysis under VUV radiation (λ=185 nm) and
photocatalysis in the presence of HPW/BMS (λ = 254 nm) were taken place in the
reaction solution.
22
Acknowledgments
This work was financed by the National Natural Science Fund of China
(Foundation of Guangdong Province of China; No. U1401235) and the Basic
Scientific Research Business Funded Projects of South China University of
Technology, China (No. 2017ZD068).
23
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27
Highlights
Defluorination of PFOA in VUV/HPW/BMS system was firstly demonstrated.
VUV/HPW/BMS system had better defluorination effect than VUV and VUV/HPW
systems.
PFOA concentration, HPW/BMS dosage and initial pH affected PFOA degradation.
28
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